1. Field of the Invention
The invention relates generally to a system and method for vehicle battery protection, and more particularly to battery under-charge and battery over-charge protection for series and series-parallel hybrid electric vehicles.
2. Background Art
In general, a hybrid electric vehicle (HEV) has four major power generation/conversion components. The power generation/conversion components include an internal combustion engine, a generator, an electric machine that performs as a motor in one mode of operation and as a generator in another mode of operation and a battery (or other appropriate energy storage unit such as an ultra capacitor in some cases). There are basically three different configurations for the HEV, depending on how the engine, generator and electric machine are connected. The three types of HEV configurations are series HEV, parallel HEV and series-parallel HEV.
During the normal drive mode of vehicle operation, the generator (except the case of parallel HEV) will convert mechanical energy into electrical energy and supply the electrical energy to the electric machine and battery. The electric machine will turn the electrical energy back to mechanical energy to drive the wheels. For the engine start operation, however, the generator will perform as a motor and will consume the battery energy as does the electric machine. Conversely, the electric machine will perform as a generator during braking (i.e., regenerative braking) to convert vehicle kinetic energy into electrical energy and charge the battery using the electrical energy generated by regenerative braking. The electrical power generated/consumed by the generator and the electric machine is controlled by a vehicle control system.
One example of a conventional approach to battery protection for hybrid electric vehicles is the system and method disclosed in U.S. Patent Application 2003/0052650. A hybrid control module determines various discharge and recharge allowances and limits for operation of the battery with different operating conditions of the hybrid electric vehicle. It also determines various periods of time and limits operation of the battery with different operating conditions of the hybrid electric vehicle. The allowances and periods of time may be used as limits that are imposed on commands for the motor.
However, the battery may be over-charged or over-discharged when the battery limits (e.g., for current, voltage or power) may not be considered properly in calculating the generator power command (P*gen) and the motor power command (P*mot) using such conventional approaches. The battery over-charge, over-discharge or over-power may cause battery damage and result in vehicle shut down, or quit on road (QOR) conditions, and potential user dissatisfaction.
It would be desirable to have a system and method for a battery protection strategy for hybrid electric vehicles that reduces or eliminates battery over-charge and battery over discharge conditions.